The drydocks are above sea level. The triangle section is a ship lift: you push the ship into the center channel, which is deep draft at sea level. A gate caps the tip of the triangle, and water is pumped into the triangle. This lifts the ship up about 25 feet. One of the three docks is then pumped full of water also, and the gate across the dock is opened to allow the ship to float into it. The dock and triangle are allowed to drain, and the ship is left high and dry above sea level.
The triangle design allows a single ship lift to service all three docks, and the sloped bottoms reduce the amount of water that need to be pumped in.

If you mean the largest vessel lifted at Lisnave, the drydocks are Panimax size, and routinely dock ships up to 985 ft x 106 ft x 39.5 feet, about 65,000 tons.

If you mean the largest ship lift in terms of lifting height, it is the one at Scharnebeck that lifts ships up 38 meters, though it can only lift a vessel of 105.4 x 15.8 x 3.4 metres (about 1300 tons). The one under construction in China at the Three Gorges Dam will lift a 280 x 35 meters x 5 meters (about 3000 tons) vessel up a height of 113 meters.

I got to see Dry Dock 12 at Newport News in person one time. It boggled the mind: 2,170 feet long and 250 feet wide...it was unreal. Especially considering that there are several larger than that in the world.

Quote:Well, there's the biggest dry dock in the world at Harland & Wolff in Belfast...

556m (1824 ft) by 93m (305 ft), with a capacity of 1.2 million tonnes according to Wikipedia.

Lisnave's Margueira (near Lisbon) yard (closed a few years ago) had what was for a long time the largest, the Alfredo da Silva Dry Dock, finished in 1970. It had a capacity of 1 million tonnes. Can´t find the exact dimensions but you can see it here.